[Immunologic relationship between the pituitary glycoprotein hormones of fishes and mammals as well as their alpha and beta subunits]

Properties of common carp gonadotropin I and gonadotropin II

Two gonadotropins, GtH I and GtH II, were extracted with 35% ethanol-10% ammonium acetate, pH 6.1, from female common carp pituitary glands and purified by ion-exchange chromatography on a DE-52 column followed by gel filtration on a Sephadex G-75 column. Molecular weights of GtH I and GtH II as determined by SDS-PAGE were 45,000 and 35,000, respectively. Both GtHs dissociate into two subunits following reduction with beta-mercaptoethanol. These subunits contain different N-terminal amino acids (Tyr and Gly for GtH I; Tyr and Ser for GtH II). GtH I was acid stable and did not dissociate into subunits following treatment with 0.1% trifluoroacetic acid; GtH II readily dissociated into subunits by this treatment. GtH I and GtH II have distinct elution profiles on reverse-phase HPLC. The N-terminal amino acid sequence of the beta-subunit of GtH II was identical to that of common carp maturational GtH described by other workers suggesting that GtH I is a newly identified molecule. This was supported by radioimmunoassay analysis. GtH II and a common carp maturational GtH preparation (F11 cGtH; Peter et al., 1982, J. Interdiscipl. Cycle Res. 13, 229-239) had similar immunological activity in tests with antisera to the beta-subunit of maturational GtH whereas GtH I had low (less than 6%) cross-reactivity. GtH I, GtH II, and F11 cGtH were equipotent in tests with antisera to the alpha-subunit of maturational GtH suggesting these molecules contain a similar alpha-subunit. In vitro bioassays using goldfish revealed that GtH I and GtH II share the same spectrum of biological activities causing stimulation of ovarian and testicular steroidogenesis and induction of oocyte final maturation. The demonstration of two chemically distinct GtHs in common carp is similar to what has been described for chum and coho salmon.

Purification of tilapia thyrotropin from a crude pituitary homogenate by immunoaffinity chromatography using a matrix of antibodies against porcine follicle-stimulating hormone

An immunoadsorbent matrix using antibodies against porcine follicle-stimulating hormone (pFSH), a high heterothyrotropic stimulant in tilapia, was used to purify tilapia thyrotropic hormone (t-TSH) from crude pituitary extracts. A homologous bioassay monitored TSH bioactivity during the purification. Thyroid hormones (thyroxine, T4; triiodothyronine, T3; and reverse triiodothyronine, rT3) and testosterone were measured in vivo in Tilapia nilotica. TSH activity eluted as one major peak at pH 2.8 using a PBS-glycine buffer. The TSH fraction increased plasma T4 and plasma rT3. The potency of tTSH was comparable to that of pituitary extract or its Con A II fraction; however, pFSH was a stronger thyroid stimulant. tTSH had no effect on plasma T3 levels and was free of gonadotropic activity, as indicated by its failure to alter plasma testosterone concentrations. Chromatographic and electrophoretic analyses demonstrated a high degree of purity. Like other vertebrate TSHs, the tTSH appeared to have a subunit structure with a possible microgeneity in one subunit.

Stimulation of thyroid function by several pituitary hormones results in an increase in plasma thyroxine and reverse triiodothyronine in tilapia (Tilapia nilotica)

In this study, intravenous injection of several doses of porcine follicle stimulating hormone (pFSH: 0.002, 0.01, 0.05, and 0.5 micrograms/g body wt), bovine TSH (bTSH: 0.5 micrograms/g body wt), and ovine growth hormone (oGH: 0.04, 0.02, and 0.4 microgram/g body wt) stimulated an increase in plasma thyroxine (T4) and reverse triiodothyronine (rT3) in tilapia. This effect occurred in a dose-dependent manner. pFSH was the most potent in stimulating thyroid function. The dose of 0.002 microgram pFSH/g body wt increased plasma levels of T4 over control levels (2.59 +/- 0.16 ng/ml) about 2.5-fold within 4 hr, whereas a concentration of 0.5 micrograms/g body wt caused a great and prolonged increase of T4 and rT3 levels. Control levels (2.59 +/- 0.16 ng/ml for T4 and 40.37 +/- 8.60 pg/ml for rT3) were increased 19- and 22-fold respectively, over 24 hr. An increase of T4 and rT3 levels occurred also after injection of total hypophyseal extract and Con A II glycoprotein fraction of a tilapia pituitary homogenate, whereas the protein fraction failed to alter plasma concentrations of T4 and rT3. rT3 levels were also significantly increased at 2 hr, but not at 1 hr, after injection of T4. Basal T3 levels (1.90 +/- 0.22 ng/ml) were reduced by half over 24 hr in all experiments. These results suggest the existence, in tilapia, of a 5-D pathway deiodination of T4 which is pituitary independent. Stimulation of T4 release is always followed by an increase in plasma rT3 levels.

Immunocytochemistry of gonadotropic cells in the pituitary of Pomadasys jubelini (teleost fish)

An antibody specific for the beta subunit of carp gonadotropin was applied to pituitary sections from Pomadasys jubelini in order to characterize and localize the gonadotropic cells by immunocytological means. It reveals the gonadotropic zone lying mainly at the periphery of the proximal pars distalis with lateral extensions around the pars intermedia. The separation between the pars proximalis and the pars intermedia is irregular. Short finger-like extensions of the gonadotropic zone protrude more or less into the pars intermedia. The immunocytological results corroborate previous tentative identifications of the gonadotropic cells by classical cytological methods and suggest that the gonadotropic hormone produced is immunologically related with several previously studied fish gonadotropins.

Cloning and sequence analysis of the cDNA for the pituitary glycoprotein hormone alpha-subunit of the European eel

A cDNA library constructed using mRNAs isolated from pituitary glands of estradiol-treated eels was screened with a cDNA fragment for the rat glycoprotein hormone alpha-subunit. Three out of 10,000 cDNA clones were revealed and subcloned in pUC13 for characterization and sequencing. All three had the same nucleotide sequence except for a single, silent change in the coding sequence for one of them, and for the location of the poly(A) tail. Analysis of the deduced amino acid sequence strongly suggests that these cDNA clones encode the precursor for the eel common glycoprotein hormone alpha-subunit. This precursor would therefore consist of a 93 amino acid apoprotein preceded by a 24 amino acid long signal peptide. Alignment with glycoprotein hormone alpha-subunits from fish and mammals reveals high homology, ranging from 60 to 90%. Particularly, the ten cysteines and the two putative N-linked glycosylation sites were at the same position. Comparison between fish and mammals shows also that two regions are highly conserved, comprising about half of the protein length. This high conservation rate through evolution argues for the importance of these regions in the conservation of biological properties of the alpha-subunits. In contrast, other regions are highly variable and could be responsible for the immunological specificity. Northern blot analysis of pituitary RNA from control and estradiol-treated eels showed that estradiol treatment strongly increases the pituitary content of mRNA encoding the glycoprotein hormone alpha-subunit.

Immunohistochemical investigation of the pituitary of the sturgeon (Acipenser baeri, Chondrostei)

An immunohistochemical study of the sturgeon (Acipenser baeri) pituitary was undertaken using antisera directed against hormones from various classes of vertebrates, including the only pituitary hormone available from sturgeon, gonadotrophin. A positive reaction was obtained after application of antisera towards the following hormones 1-24 synthetic ACTH (1-24 ACTH), melanophore stimulating hormone (MSH), ovine prolactin (oPRL), ovine growth hormone (oGH), salmon growth hormone (sGH), carp gonadotrophin (cGTH) and its beta subunit (βcGTH), sturgeon gonadotrophin (aciGTH), carp thyrotrophin (cTSH) and β subunit of the human thyrotrophin (βhTSH). The results demonstrate that, in general, the sturgeon pituitary resembles that of teleosts as regards the distribution of the different cell types: ACTH and PRL cells in the rostral pars distalis, GTH, TSH and GH cells in the proximal pars distalis and MSH and PAS-cells in pars intermedia. In addition to the topographical organization of the sturgeon pituitary, this study provides data on the immunological relationships between sturgeon pituitary hormones and those of other vertebrates.

Purification of gonadotropic hormone from the pituitary of the African catfish, Clarias gariepinus (Burchell), and the development of a homologous radioimmunoassay

Glycoprotein gonadotropic hormone (GTH) was purified from 2000 pituitaries of male and female African catfish, Clarias gariepinus. The first step was chromatography on concanavalin A-Sepharose followed by filtration on Ultrogel Aca 54, chromatography on DEAE-cellulose, and filtration on Ultrogel Aca 54, respectively. Finally, the purified fractions were analyzed by polyacrylamide gel electrophoresis. The gonadotropic activity in the different fractions was characterized using two tests: the radioimmunoassay for carp gonadotropin-beta subunit was used to quantify the immunoreactive GTH and a cAMP accumulation test was applied to measure the GTH biological activity. The purified glycoprotein GTH was used to raise antibodies and to develop a radioimmunoassay. This resulted in an assay with a variation between assays of approximately 4%, a precision of 4-8%, and an accuracy of 4-8%. GTH levels can be measured over a range of 0.8 to 12.5 ng/ml.

Effects of steroid hormones on pituitary immunoreactive gonadotropin in European freshwater eel, Anguilla anguilla L

The low pituitary immunoreactive gonadotropin (IR GTH) contents of immature female or male freshwater eels were strongly increased after intraperitoneal injections of estradiol. Using female eels, the effect was shown to be dose dependent. In neither sex was modification of pituitary IR GTH observed after progesterone treatment; a very small increase (significant in females but not in males) was produced by cortisol. An important sexual difference occurred with testosterone, which strongly stimulated pituitary IR GTH level in male but not in female eels. The possibility of a low central aromatase activity responsible for the lack of effect of testosterone in the female eel is discussed. Circulating IR GTH levels were very low in steroid-treated eels, as in controls, indicating a weak GTH release. Thus, as in juvenile, immature fish from other species, a sexual steroid stimulated eel pituitary GTH content; the peculiarity of the apparent "blockade" of eel sexual development appears not related to a lack of sensitivity of gonadotropin synthesis to the positive feedback of a sexual steroid.

Development of a heterologous radioimmunoassay for eel (Anguilla anguilla) gonadotropin

A heterologous radioimmunoassay (RIA) for eel gonadotropin (GTH) was developed, using an antiserum to the beta subunit of carp GTH (cGTHbeta). Partially purified eel GTH preparations or crude eel pituitary extracts produced partial cross-reactions: the inhibition curves reached a plateau before complete inhibition of the binding of the tracer and the slopes were lower than those for cGTH. However, the parallelism between the inhibition curves produced by all the eel samples allowed estimation of their GTH activity relative to an eel GTH standard; under these conditions, the RIA exhibited good reproducibility within and between assays. Three different RIA systems for cGTHbeta using different antisera gave similar estimations of immunoreactive GTH (irGTH) content of eel GTH preparations and pituitary extracts. The specificity of teh heterologous RIA for eel GTH was first assessed by comparing RIA and bioassay data: as with carp pituitary extracts, no significant difference was found between immunological and biological estimations of GTH activity for partially purified or crude pituitary extracts from normal female eel. Further validation of the specificity of the RIA for eel GTH was provided by the variation of the eel pituitary irGTH content under different hormonal treatments: a significant increase in female eel pituitary irGTH content was produced by treatment with estradiol-17 beta or with carp pituitary extract, a result in agreement with previous histological reports of Olivereau and collaborators. The heterologous RIA for eel GTH, in view of its sensitivity, reproducibility, and specificity, is a useful tool for physiological studies in the eel.